The overall objective of this proposal is to examine the mechanisms of control of erythropoiesis concentrating on those that, when disturbed, result in polycythemia. We have described the disease entity of primary familial and congenital polycythemia (PFCP), which provides a model for studying the alterations of the control of erythropoiesis by augmented erythropoietin (Epo) activity. The central premise for our studies is that those mutations leading to a disease phenotype must cause disturbances in non-redundant cellular or metabolic pathways. Identification of these mutations and elucidation of their effect will lead to a) accurate diagnoses, b) eventual curative strategies of these and related disorders (such as leukemias), c) improved understanding of hematopoiesis in health and disease. Our cumulative experience with PFCP has led to the conclusion that PFCP may be associated with an increased risk of stroke and other cardiovascular abnormalities. These observations can now be elucidated in an animal model bearing the human disease causing gain-of-function erythropoietin receptor (EPOR) mutation that is amenable to tissue specific reactivation of normal and augmented Epo signaling. These studies should elucidate the role of Epo signaling in non-erythroid tissues. Further, we have also shown that in most affected families polycythemia is not due to EPOR mutations. To achieve our research objectives we will pursue these Specific Aims: Aim 1. Determine the genetic and molecular basis of PFCPs, la. Employ genetic mapping and a combined functional/positional approach to identify the locus for PFCP families in which EPOR is not mutated, lb. Using our mouse model, which bears a human PFCP gain-of-function erythropoietin receptor (EPOR) mutation we propose to create mice with tissue-specific selective EpoR expression and study the effect of EPO signaling in non-erythroid tissue. Aim 2. Determine the molecular basis and phenotype for congenital polycythemias characterized by elevated Epo concentrations and VHL mutations (Chuvash polycythemia phenotype). 2a. Define Von Hippel Lindau syndrome (VHL) mutations associated with a Chuvash polycythemia-like phenotype in non- Chuvash ethnic groups. 2b. Study the effect these polycythemia-inducing VHL mutations on hypoxia-sensing and on carcinogenesis, coagulation pathways, and endothelial function. Aim 3 Determine the genetic and molecular basis for congenital polycythemias characterized by elevated Epo concentrations and mutations of other than the VHL gene.